That's because planets need to be the perfect distance from their star so they're not too hot or too cold for creatures to live on them.

The Kepler telescope will search for the planets by looking for any dimming of the light from the stars as planets pass between them and the spacecraft.

But it won't be easy.

James Fanson, who's managing the Kepler project, said: "Trying to detect Jupiter-size planets crossing in front of their stars is like trying to measure the effect of a mosquito flying by a car's headlight.

"Finding Earth-sized planets is like trying to detect a very tiny flea in that same headlight."